1. Field of the Invention
This invention relates to a twisted nematic liquid crystal device and, more particularly, to a twisted nematic liquid crystal negative display element of the light transmitting type having improved color tone and contrast through the addition of a positive dichroic die to the host nematic liquid. It has particular application to negative display type liquid crystal devices, which have zero to low transmittance in a non-selected region and a high transmittance in selected regions of the display, thereby effecting a white or light appearance in the selected region against a dark background.
2. Description of the Prior Art
Negative display liquid crystal devices using the twisted nematic effect, i.e., field effect liquid crystal display elements, include a nematic liquid crystal having a positive dielectric anisotropy, in a twisted orientation between upper and lower substrates and a pair of polarizing plates. Materials having a positive dielectric anisotropy orient in an electric field with the molecules preferentially parallel to the field and are used in field effect electro-optic devices.
In a typical device upper and lower substrates are spaced apart and a positive dielectric anisotropic nematic liquid crystal is disposed between the substrates. Generally, the liquid crystal between the upper and lower substrates is effectively twisted by an angle of about 90.degree. by rubbing or otherwise treating the substrate surfaces to provide parallel orientations on the substrates and thereafter disposing the substrates so that these orientations are at an angle of 90.degree. to each other. Polarizing plates are disposed between the upper and lower substrates so that the liquid crystal is disposed therebetween.
A negative display may be obtained if the polarizing axis of the lower polarizing plate coincides with the direction of orientation of the liquid crystal molecules contacting the lower substrate and with the polarizing axis of the upper polarizing plate. In an activated portion of the liquid crystal material, the liquid crystal molecules are brought into a perpendicular position with respect to the substrates, i.e. they are not twisted. The rotatory polarization disappears and this portion of the liquid crystal cell has a high light transmittance because the polarizing axes of the upper and lower polarizing plates coincide with each other. In the non-selected portion, the light transmittance is low because the liquid crystal twists the light perpendicular to the axes of the polarizing plates. As a result, the negative display liquid crystal device transmits light while non-selected portions of the liquid crystal display do not allow the light to pass.
One problem encountered in this arrangement results from leakage of light into the non-selected portion of the display element. Leakage impairs the quality of the display since the non-selected portion fails to completely shield the light. Additionally, unevenness in color tone resulting from light path retardation is another problem encountered. Retardation is also known as optical-path difference. In a liquid crystal display element the light is split into polarized beams at right angles to each other. Each beam of light has its own refractive index. The refractive index anisotropy is the difference between these two indices. In a liquid crystal display element the retardation is dependent upon the value of the refractive index anisotropy and the cell thickness. That is, the relation between them is formulated by R=.DELTA.n.multidot.d, wherein R is retardation, .DELTA.n is refractive index anistotropy, and d is substance thickness. Thus, with reference to the above-described general definition of retardation with respect to an optical path, it can be seen that as a consequence of the liquid crystal splitting an incident beam into two beams polarized at right angles to each other and having separate refractive indices, that the two beams will not propagate through the liquid crystal medium at the same rate. As a consequence, even slight differences in cell thickness between adjacent portions of a liquid crystal cell will give rise to differences in the length of the optical path of the perpendicular light waves. Such differences cause adverse effects on background contrast, color tone and also lead to leakage of transmitted light into a non-selected area. Such variations in cell thickness are a recurring problem as a consequence of the need to apply force to the liquid crystal elements during fabrication of a liquid crystal display device, as well as a lack of uniformity in cell thickness. For example, the dashboard panel for automobiles is large in size. Uniform cell thickness in a display area is very difficult to maintain, even with the use of spacers, because of the large area involved, and also the need to apply pressure over a large area of substrates to fabricate the liquid crystal cells.
Accordingly, it is desirable to provide an improved liquid crystal display device which overcome these problems and provide a display of improved contrast and color tone.